All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If your mother had made a will about anything from this inheritance, then it is not permissible for your father to conceal this; rather, he should show it so that he would not prevent someone from his right or give someone else what he does not deserve. However, it should be noted that a will to an heir is void unless all other heirs consent to it, and provided they are all adult and have a sound reasoning. It is confirmed that the Prophet said: "There is no will for an heir." [Al-Bukhari and Muslim] For more benefit, please refer to Fataawa 116588 and 119243.
Moreover, your father is not permitted to prevent any heir from his share in the inheritance, and we do not know how he claims that this money is his property. If it is confirmed that he gave his other wife any money from the inheritance, then he is indeed wrong doer and a transgressor, and she is not permitted to take anything from it. In any case, there is no doubt that your father is very wrong if it is confirmed that he really did what you attributed to him.
Therefore, we advise you to talk to your father in a good and gentle manner about these matters which you mentioned and you may seek the help of some relatives and pious people in this regard. If he agrees to divide the inheritance, then this is what is required, otherwise if it is not possible to solve this problem except by taking him to court, then this is permissible and this is not considered as cutting ties with kinship.
Abu Yazeed Ma’n ibn Yazeed narrated: "My father Yazeed had taken some Dinars (gold coins) for charity and kept them with a man in the mosque (so that he would give them to the poor) but I went and took them and brought them to my father." Then my father said: "I swear by Allaah! I did not intend to give them to you." Therefore, I took the case to the Prophet . On that the Prophet said: “O Yazeed! You will be rewarded for whatever you intended; and O Ma’n! Whatever you have taken is yours.” [Al-Bukhari]
Ibn Hajar said: "This is evidence that it is permissible for a father to take his son to court and vise-versa, and that this, by itself, is not considered as cutting ties with kinship." In any case, you should accompany your father with kindness.
Finally, if a father died and left behind a wife, three children (sons and daughters), then the wife gets one-eighth because of the existence of children, and the remaining of the inheritance will be divided between the children, the male twice the share of the female. However, if all the children are females, then they get two-thirds. Whatever money remains after the heirs [had taken their share] will be given back to the three girls and not to the wife. Ibn Qudaamah said: “The remainder of the inheritance will be given back to the heirs whose legal share is determined by the Islamic Law according to their share in the inheritance except the husband and the wife.”
Allaah Knows best.